Carbonates are important carriers and subjects of study in the deep carbon cycle of the Earth.Fluorine-bearing rare earth carbonates(such as bastnäsite),which are common ore minerals in rare earth deposits within orogenic belts and continental margins,have simi- lar crystal structures to hydroxyl-bearing rare earth carbonates and often form solid solutions in varying proportions.This research employs a combination of high-temperature deuterium tracer experiments,single-crystal X-ray diffraction,infrared,and Raman spectroscopy to define  the  conditions  under  which  deuterium  substitution  occurs  in  bastnäsite  from  an  experi- mental    mineralogy    perspective.It     investigates    the    structural     state,thermal    stability,and conditions  for  surface  fluorine  substitution  by  hydroxyl  groups  in  bastnäsite  at  different tem- peratures   and   during   the   deuterium   substitution   process.The   results   show   that   bastnäsite undergoes  deuterium  substitution  at  548  K  and  under  a  CO₂gas  flow  rate  of  280  ml/min.  The  structural type remains unchanged during temperature increases in the range of 293 K to 548  K,but partial  decomposition  occurs  at  573  K  and  598  K.After  storing  the  samples  at  la- boratory  ambient  temperature  and  pressure  for  six  months  following  the  deuterium  substitu- tion  experiment,it  was  found  that  the   surface  fluorine  and  hydroxyl  groups  underwent  iso- morphous   replacement,forming   hydroxy-fluorocarbonates,and   that   there   was   adsorbed   wa- ter  on  the  surface.Bastnaesite  are  products  of  mineralization  involving  carbon,fluorine(-hy- droxyl),and   rare   earth    elements   in   the    Earth's   near-surface   environment.The   results    of this  experiment  are  significant  for  understanding  the  geochemical  cycling  and  enrichment mechanismsof    carbon,water,fluorine,and     rare    earth     elements.